This invention relates to copolymeric resinous siloxanes containing SiO.sub.2 units, trimethylsiloxane units, and alkenyldimethylsiloxane units. The resinous siloxanes are useful in silicone coatings to provide intermediate range release properties for adhesives.
The application of curable silicone compositions to substrates such as paper to aid in the release of adhesive materials therefrom is old and well known in the coatings art. For example, the preparation of laminates consisting of a release paper and a sheet material which may be a decorative lamina or a label material coated with a pressure sensitive adhesive is well known. When it is desired to use the coated sheet material and finally affix it, the release paper is stripped off and discarded.
In many uses, however, it is possible for the release capability of a substrate to be too great. For example, pull-away backings for adhesive tapes must separate without difficulty from the adhesive which they protect, but they must not contain such effective release agents that they accidentally fall off or slide away. Such occurrences are possible with some presently known silicone release agents.
In order to obtain intermediate range release values, Keil, U.S. Pat. No. 3,527,659, teaches the use, in a silicone release composition, of a copolymer of R.sub.3 SiO.sub.1/2 and SiO.sub.2 units having no more than 0.6 weight percent silicon-bonded hydroxyl groups, with R denoting a monovalent hydrocarbon radical of no more than 2 carbon atoms. More specifically, Sandford, U.S. Pat. No. 4,123,604, teaches the use of a similar copolymer of R.sub.3 SiO.sub.1/2 units, SiO.sub.2 units, and (CH.sub.2 .dbd.CH)R.sub.2 SiO.sub.1/2 units with R denoting a monovalent hydrocarbon radical of no more than two carbon atoms. Sandford teaches that the use of the copolymer in silicone release compositions curable by the addition of methylhydrogenpolysiloxanes to silicon-bonded vinyl groups provides intermediate range release characteristics.
Although the conventional siloxane release coating compositions normally perform satisfactorily in use, there is a continuing demand for improved compositions. In particular a need has been expressed for compositions which cure faster and which require the application of less energy for the curing step. In response to this need, radiation curable coating compositions such as the mercapto organosiloxane and methylvinylsiloxane combinations disclosed by Michael, et al., U.S. Pat. No. 3,873,499; Gant, U.S. Pat. No. 4,064,027; and Bokerman, et al., U.S. Pat. No. 4,052,529 were developed. However, these compositions met resistance in the market place because they emitted an obnoxious odor after the cure process. The odor was quite noticeable, especially when the compositions were used in thin layer applications, such as in a paper coating process.
In order to reduce the odor associated with radiation cure of these types of coating compositions, Gordon, et al., U.S. Pat. No. 4,107,390, teach that the polymethylvinylsiloxanes in the compositions can be replaced by 1,1'-oxy-bis(1-methyl-1-silacyclopentene). Takamizawa, et al., G.B. Pat. No. 1,569,681, further teach UV curable compositions comprising (a) an organopolysiloxane having in the molecule at least one mercapto-containing organosiloxane unit, (b) an organopolysiloxane having in the molecule at least one allyl-containing organosiloxane unit, and (c) a photosensitizer. Although the odor associated with curing the compositions of these patents is not as marked as that with the vinyl-containing compositions, the unsaturated components used in these compositions are difficult and relatively expensive to prepare.
In still another attempt to reduce the odor associated with radiation cure of siloxane coating compositions, White, et al., copending U.S. patent application Ser. No. 713,940 which is owned by the assignee of this patent application, teach UV curable compositions comprising (a) mercapto group containing organosiloxane, (b) an organosiloxane having at least two siloxane units bearing cyclohexenylethyl substituents (C.sub.6 H.sub.9 CH.sub.2 CH.sub.2 --), and (c) a photoinitiator.
There is a need for improved components for use in silicone release compositions, especially the radiation curable compositions, to provide intermediate range release for these systems. At the same time, it is desirable that any new component allow even more rapid and efficient cure without emission of obnoxious odor.